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The Joy of Extreme Possibility

Nuclear rocket designs are hardly new. In fact, it was clear as early as the 1950s that conventional chemical rocketry was inefficient, and programs like Project Rover, set up to study the use of nuclear reactors to heat liquid hydrogen for propulsion, aimed at the kind of rockets that could get us beyond the Moon and on to Mars. The NERVA rocket technology (Nuclear Engine for Rocket Vehicle Application) that grew out of all this showed great promise but ran afoul of political and economic issues even as the last Apollo missions were canceled. Nor is the public wariness of nuclear methods likely to vanish soon, yet another hurdle for future ideas.

But making people aware of what has done and what could be done is good practice, as Kenneth Chang does by example in his recent piece on the 100 Year Starship Symposium, which bears the optimistic title Not Such a Stretch to Reach for the Stars. In interstellar terms, propulsion is the biggest problem of all. Chang’s article suggests a pathway through conventional rocketry and into nuclear-thermal designs, with reference along the way to using nuclear engines to generate the electrical fields that power up an ion engine. The goal on this pathway is fusion, though Chang admits no one has yet built an energy-producing fusion reactor.

The Daedalus concept was fusion-based, and the ongoing Icarus project that followed is now examining Daedalus to note the effect of thirty years of new technology. But Chang has also talked to James Benford, whose interest in laser and microwave beaming remains strong. Leave the propellant behind and you’ve maximized payload, in addition to working with known physics and apparently achievable engineering. And there continue to be startling new concepts like those of Joseph Breeden, who finds a more extreme way to create an engineless vehicle:

From his doctoral thesis, Dr. Breeden remembered that in a chaotic gravitational dance, stars are sometimes ejected at high speeds. The same effect, he believes, could propel starships.

First, find an asteroid in an elliptical orbit that passes close to the Sun. Second, put a starship in orbit around the asteroid. If the asteroid could be captured into a new orbit that clings close to the Sun, the starship would be flung on an interstellar trajectory, perhaps up to a tenth of the speed of light.

“The chaotic dynamics of those two allow all the energy of one to be transferred to the other,” said Dr. Breeden, who came toting copies of a paper describing the technique. “It’s a unique type of gravity assist.”

What I call the ‘joy of extreme possibility’ has animated interstellar studies since the days of Robert Forward. It works like this: We know the distances between the stars are so vast as to dwarf the imagination. Indeed, most people have no notion of them, seeing an interstellar mission as merely a next step once we have explored the outer system, a kind of juiced-up Voyager. The scientists and engineers who work on these matters, knowing better, realize how far beyond our current technologies these journeys really are. So they’re not afraid to speculate even at the absolute far end of the plausible (and often beyond that). Work your way through interstellar papers like these and you pick up an infectious, jazzy brainstorming. It’s the kind of mental riffing on an idea that a John Coltrane or a McCoy Tyner does with a musical theme.

And by the way, Chang is careful to get those distances across to readers. I’m always interested in homely comparisons because you can use them to boggle audience minds when speaking about interstellar flight. This is useful, because a boggled mind often becomes a curious one, and while you can never predict these things, occasionally interstellar studies gain a new adherent. Chang cites a Richard Obousy analogy: If the Earth were Orlando and Alpha Centauri were in Los Angeles, then the Voyager spacecraft would have traveled but a single mile.

Even after all these years, that one still boggles my own mind. Chang again:

Another way of looking at the challenge is that in 10,000 years, the speed of humans has jumped by a factor of about 10,000, from a stroll (2.6 m.p.h.) to the Apollo astronauts’ return from the Moon (26,000 m.p.h.). Reaching the nearest stars in reasonable time — decades, not centuries — would require a velocity jump of another factor of 10,000.

It’s good to see the 100 Year Starship Study steadily percolating in the news. Maybe one day these concepts will not seem as esoteric as they do today. I note as I write this, for example, that my word processor flags the word ‘starship’ as a spelling error. We need to set deeper roots into the culture than that. We can start by doing what conference organizer David Neyland told Chang he wants to do, to establish a bar high enough that people “will actually go start tackling some of these really hard problems.” Of course, the real bar is set by nature, and it’s the highest bar we as a species have faced in terms of travel times and distance. But the joy of extreme possibility only ignites the spirit when everything is on the table and the challenge is immense.


Comments on this entry are closed.

  • David October 19, 2011, 10:21

    I recall there was a post under the conference that wondered if Project Orion came up, I do too. I dont know if the Wikipedia write up is correct but it said 8% is possible. We have a lot of propulsion available….

  • Tony P October 19, 2011, 10:39

    Is there any research being done to investigate a nuclear powered ion engine ship like the one at the heart of Project Prometheus? http://en.wikipedia.org/wiki/Project_Prometheus

    The project was cancelled officially in 2005, but I don’t think people are looking at NASA for any technological leap any time soon. I assume a ship like Prometheus would need to be built in high orbit and have some sort of assistive thrust to get it going before the low thrust (but high specific impulse) ion engines kick in to take over.

    Also, worth keeping an eye on Bussard’s new fusion reactor design in the works. I don’t know how far along they are, but it would revolutionize space travel and terrestrial power generation. My biggest concern is the scarcity of helium-3, which I believe is the main fuel for the reactor, but is abundant in Jupiter’s atmosphere. My concern may be the biggest drive to pursue better deep space craft, though. Specifically extracting the helium 3 from Jupiter with mining platforms in high orbit outside of Jupiter’s deadly magnetosphere and ferrying huge tanks filled with fuel that are then dropped into the ocean and retrieved with sea ships. I know, I’m a wild dreamer. But this could be the start of a booming interplanetary economy.

  • James Benford October 19, 2011, 11:09

    The Chang quote you use is actually an exact quote of one of my analogies. I used two analogies to open my first session of Starship Concepts on Friday. The entire quote is:

    “If the length of the room we’re in is the E-M distance, about 60 ft, then Alpha Centauri is not on Earth at all, but beyond the moon. In fact it’s further, about 4.9 times the mean moon distance.

    One ten-thousandth of the 26,000 mi/hr of Apollo is 2.6 mi/hr, the speed of a stroll, which is what we did in pre-history. 10,000 times 26,000 mi/hr is 2.6 10^8 mi/hr. Speed of light is 6.6 10^8 mi/hr. So 10,000 times the fastest we’ve done is (2.6/6.6 =) 0.4 c. That’s about the fastest starship anyone’s proposed. So, as a ratio, we have as far to go as we have come!

  • Martin J Sallberg October 19, 2011, 12:32

    Some other extreme possibilities: study local anomalies in the expansion of the Universe and use them as shortcuts, manipulate the Brownian motion in a way that allows quantum superposition of large objects (including astronauts), or explore how the new class of supraconductors (class 1,5) can upset the laws of physics.

  • ljk October 19, 2011, 15:43

    Here are some other celestial scale comparisons:

    The Carl Sagan Planet Walk in Ithaca, New York, dedicated on his birth date (November 9) in 1997, is a 3/4-mile scale model of the Sol system from our yellow dwarf star to Pluto. They are planning to put a marker for Alpha Centauri at the same scale. It will be located in Hawaii.

    If you could shrink the Milky Way galaxy down to where you could hold the entire Sol system in the palm of your hand, the galaxy would be the size of North America.

  • Nathan Currier October 19, 2011, 19:30

    The NY Times lost something in translation. There’s no way could you get to 10% of lightspeed using a gravitational slingshot involving our sun and an asteroid. The energy might be there in the asteroid’s orbit but the asteroid’s gravity is too weak to give the spaceship that much of a boost. (Alternatively, you could say that the spaceship’s gravity is too weak to significantly shrink the asteroid’s orbit). If you want 10% of lightspeed from a gravitational slingshot, you need a much more energetic system. It works for tight binary white dwarves (not the porno movie). Such systems could be the crossroads of the galaxy. Would need modular colony ships that can separate into small pieces, each with good armor.

  • Astronist October 19, 2011, 20:23

    I’m skeptical about the Breeden concept. Given the bodies at hand in our Solar System, which asteroid would he use? Then he needs to divert another asteroid onto an orbit which intercepts the first one (I presume that’s what he’s suggesting) at perihelion and circularises its orbit there (I don’t see how else he can do this). But asteroids take a lot of propulsion to budge because of their large masses. How much faster does he claim his starship will be ejected by this mechanism, as opposed to using the same amount of propulsion on the starship directly in the normal manner? In any case, the concept will be useless to capture the vehicle on arrival, so it’ll have to have rocket propulsion in any case in order to decelerate.

  • Astronist October 19, 2011, 20:29

    Tony P: the Bussard design is for a fusion reactor fuelled with hydrogen and boron-11. The design you are thinking of, which reacts deuterium with helium-3 (with a tritium trigger), is the Daedalus engine design of Alan Bond and Tony Martin. Daedalus, too, came up with the concept of mining helium-3 from the atmosphere of Jupiter, but John S. Lewis argues that Uranus would be an easier source.

  • Abelard Lindsey October 19, 2011, 20:33

    The huge distances between potential habitable planets is one reason why I think the future of humanity in space is mostly that of orbital habitats rather than planets.

  • Steven Rappolee October 19, 2011, 21:31

    earlier this evening I posted some hypervolocity star papers on the 100 year starship study post from a week ago, these papers discuss the gravitational instability’s of binary star systems as they are both torn apart ( one star) and ejected from the galaxy( the other star) by the saggiterias black hole. Any relation to Dr Breeden’s asteroid world ship binary pair flung from close to the sun?

  • Christopher Phoenix October 20, 2011, 1:10

    From what I have read of nuclear rocketry, it seems that the solid core nuclear designs like NERVA are limited by the temperature that the engine can withstand- if it gets too hot, the engine will melt. Gas core designs are higher performance.

    The more extreme but far more effective option is too ride the shockwave a nuclear detonation, like Project Orion. The public is likely to respond to any attempt to utilize atomic bomb detonations to propel a spacecraft with hysteria and hostility- another reason to try to improve science education, I suppose.

    I can imagine a future where futuristic spaceships with fusion pulse engines roam the solar system- there is plenty of room for improvement with nuclear pulse engines. Project Orion was to use fission bombs and solid pusher plates with giant shock absorbers, but future designs could use fusion pulse units and magnetic fields instead. The scientist who worked on Project Orion explored the possibility of a nuclear pulse starship, which remains the only design for a starship that could be built with near-future technology.

    Rocket propelled starships would be a slow and tedious way to explore the universe. I think the key to fast star travel is finding a technique to propel a spaceship without needing propellent. Even nuclear rockets have their mass ratios spiral into the impossible when you try to reach a substantial fraction of the speed of light.

    I wish to work on speculative propulsion physics- imagine how a propellentless space drive that created thrust anywhere in the universe would impact space travel!! You could simply plug such an engine into a nuclear reactor and head off on a long space trip. Spacecraft would become much simpler vehicles. Perhaps some way of manipulating spacetime or harnessing casimr forces could lead to such a breakthrough. Yes, I know that such a drive would appear to break the Law of Conservation of Momentum- but that is the extreme possibility. At least, even if we can’t build space drives or antigravity screens, efforts to build such devices will learn something new about physics.

    Then there are the concepts that are rooted in known physics, like laser powered light-sails and Dr. Breeden’s gravity assist that could someday propel humans beyond our solar system. All these concepts are speculative- some more than others- but all of them are very exciting!!

  • Enzo October 20, 2011, 5:20

    I had this little thought nagging me for a few days now regarding a generational starship.
    Is it possible to make it leak proof for thousand of years ?
    This is less stupid you might initially think : any material lost is gone and there is no possibility of replenishing it until arrival.
    Even a tiny leak across millennia is deadly.
    Maybe some gas can be captured while traveling to offset losses. The right type of gas though ?

  • Tobias Holbrook October 20, 2011, 6:39

    I’d be more concerned about what sort of accelerations Breeden is talking about…

  • A. A. Jackson October 20, 2011, 7:48

    I too am dubious about this ‘chaotic orbital’ slingshot maneuver that gives 10 percent the speed of light.
    For one thing chaotic dynamics is highly non-linear. Even though discovered by Henri Poincaré in the late 19th century, chaos theory was only well quantified in later half of the 20th due to the appearance of the digital computer. Non linear dynamical systems are highly sensitive to initial conditions and are notoriously prone to give a horizon of unpredictability. Even setting the initial conditions of a spacecraft on a theoretical ‘chaotic’ swing-by, or combination of swing-by maneuvers (or whatever this process may be) may just well throw one into the Sun or Jupiter as as out of the solar system!
    Besides for any kind of mass 10% of the speed of light sounds like a very unlikely amount of energy to extract from the solar system.
    Attaining a fraction of the speed of light can be done, but one needs a neutron star binary or black hole binary for the maneuver, as Dyson showed a long time ago:
    Dyson, Freeman J. “Gravitational Machines.” ,Interstellar Communication, A. G. W. Cameron, Editor, New York: Benjamin Press, 1963, Chapter 12.

  • ljk October 20, 2011, 8:41

    Christopher Phoenix said on October 20, 2011 at 1:10:

    “The more extreme but far more effective option is too ride the shockwave a nuclear detonation, like Project Orion. The public is likely to respond to any attempt to utilize atomic bomb detonations to propel a spacecraft with hysteria and hostility- another reason to try to improve science education, I suppose.”

    A big YES to everything you just said above!

    Remember, folks, with Orion, you not only have a fuel supply that already exists in reality, along with real-world testing done decades ago, you also have a means for the ship’s crew to defend themselves – just in case.

  • jkittle October 20, 2011, 9:34

    note on Helium 3. Helium 3 is often compared favorably to Tritium as a fuel for fusion reactors ( in the popular press and here on these pages). Tritium can be made in practical amounts by bombarding Lithium with neutrons, which can be supplied in several ways, including fusion reactions ( if fusion were working of present day by regular old fission uranium, thorium or plutonium. The reaction that creates Tritium (Hydrogen 3) also releases a LOT of energy in and of itself. However, if helium 3 is needed it is readily obtainable from Tritium – which decays to Helium 3 with the loss of a beta particle ( 12 year half life) so.. if you replenish a pool of tritium at a constant rate and accumulate ten years worth of material, you have a pretty good source of He3, no trips to Jupiter or strip mining of the moon needed! He3 fusion [with deuterium] is not really neutron free… there are side reactions ( D+D , for example). The z-pinch machine at Sandia labs has already demonstrated the temperatures and confinement parameters needed to fuse a wide variety of light nuclei. It may be just a matter of time and engineering…
    sign me-
    Optimistic as always

  • Martin J Sallberg October 20, 2011, 11:06

    @Enzo: It would be possible to gather new material to extract gasses from on asteroids or comets en route. Indeed, it would be possible to build completely new habitats en route and split up. The classical concept of a generational ship that no astronauts can leave ignores that space is full of tiny worlds with resources. That ignorance has created completely unfounded prejudice against interstellar colonization, prejudice that pervades almost all space ethics thinktanks.

  • James Benford October 20, 2011, 12:06

    LJK says the Sagan Walk wants to put a marker for Alpha Centauri in Hawaii. I suggest that the right place for it is in the Galaxy Garden that Jon Lomberg, a long-time Sagan associate, built on the Big Island. I was there with Jon in March and can attest it’s a mind-blowing to-scale 100 ft diameter wonder! There’s a little jewel for Sol, another for Alpha Centauri. That’s where the Sagan marker should be.

  • ljk October 20, 2011, 14:35

    James Benford, that is an excellent, excellent idea. I am sure Jon Lomberg would agree.

  • ljk October 20, 2011, 14:40

    For those who would like to know more about the Galaxy Garden on Hawaii:


  • Adam October 20, 2011, 17:05

    Hi All

    Joe explained to me in an email…

    There is one important misquote in the NYT article.
    The scientific paper shows energy transfers resulting in 0.1c, but that is
    not likely to be achievable. Later in the paper it explains that achievable
    speeds extend to only around 0.04%c, or roughly 1,000 km/s. Likelihood of
    achieving a certain trajectory is done via analogy with detailed simulation
    work on hypervelocity star creation.

    …and he is working on simulations to define the envelope of the possible a bit more clearly. He has worked on globular cluster dynamics before, which is where the idea came from in the first place.

    jkittle, breeding tritium to make He3 is how it’s made for scientific use at present, since most nuclear weapons are D-T boosted and tritium decays. However the size and expense of a breeding program to make spaceship fuel would dwarf a mining operation operating in the atmosphere of Uranus. James Powell, and colleagues, designed an effective architecture for mining Uranus using near-term technology about a decade ago, but without working D-He3 reactors it’s an idea without a market.

  • Tony P October 20, 2011, 17:30

    Is there any legitimacy to the gravitomagnetic (GEM) ideas for propulsion postulated by some people, namely scientists at the ESA back in 2006. I know it’s untested with regards to moving any sort of mass around, but there was some excitement around the discovery.

  • Paul Gilster October 20, 2011, 17:46

    Tony, Martin Tajmar’s work is still interesting but the effect has not been confirmed and at the moment this one remains, so to speak, up in the air. If you’re interested in more details, our Frontiers of Propulsion Science book gets into the research, along with essays on many other propulsion topics.

  • Enzo October 20, 2011, 19:01


    Sure you can harvest comets or asteroids on route but I can see two problems :
    1) If you are half way to your destination, in the proverbial middle of nowhere,
    are there any asteroids or comets ?
    2) You are probably traveling at some 500-1000 Km/s, taking a few millennia to reach the next star. The structure you are traveling on is huge. Do you have the energy to stop it and re-start it, even if you find a suitable body ?

    The only solution (maybe) is to have some sort of funnel in front of the ship collecting volatiles, hoping to catch something else besides hydrogen atoms.
    This is not a huge problem compared to the whole generational ship business but it is not an easy one either.

  • Christopher Phoenix October 21, 2011, 0:57

    ljk said on Oct. 20, 2011 at 8:42:

    “A big YES to everything you just said above!

    Remember, folks, with Orion, you not only have a fuel supply that already exists in reality, along with real-world testing done decades ago, you also have a means for the ship’s crew to defend themselves – just in case.”

    Quite true- all the technology needed to build an Orion type ship existed in the 1960’s. Carl Sagan mentioned this on “Cosmos”, stating that we could build an Orion starship now. I think nuclear pulse propulsion would enable an age of cheap interplanetary travel.

    On the topic of nuclear weapons for space defense- nuclear weapons are actually not very efficient in space warfare. There is no blast, since there is no atmosphere. The nuke goes of like a very bright flashbulb- plenty of radiation and intense light, but no shockwave. If an alien invader was a meter from the nuke, they’d be vaporized, but the damage potential of the nuke’s flash drops off very quickly due to the inverse square law. Most of the nuclear warhead’s energy is wasted.

    The solution, ironically, comes from Project Orion. Most of an ordinary nukes energy would be wasted in a nuclear pulse drive, since little of the propellent would impact the pusher plate. The scientists found out how to tamp a nuclear warhead to focus its energy on the pusher plate, making the rocket much more efficient. The military noticed that if you made the plume a little faster and with a narrower cone, it would become a directed energy weapon. Thus the project known as Casaba Howitzer was born.

    The technology of tamping nuclear charges and focusing the blast in particular direction makes nuclear pulse drives more efficient and enables such weapons as Casaba Howitzer, nuclear pumped X-ray lasers, nuclear shaped charges, and bomb propelled kinetic weapons. So while ordinary nukes are not much good for space combat, the technology pursued in Project Orion provides many atomic space weapons that are quite deadly. An Orion battleship was in fact on the drawing boards, armed with Patriot missiles, naval guns, and Casaba Howitzer. Alien marauders beware!!!

    So- do any of you guys have thoughts on arming starships or including armed security teams on missions to other planets? Science fiction often portrays armed astronauts. What risks might real astronauts encounter? Is it logical to arm spacecraft crews, or have we just watched too many SF movies?

    What armament should be carried, if we do arm astronauts? Hand lasers with stun and kill settings, like Space 1999? Guns that shoot rocket bullets with explosive warheads? And, if we are going to be totally paranoid, what about quarantining astronauts who landed on other planets and observing them for signs of infection? I haven’t seen much discussion of this on this blog. When astronauts actually landed on the Moon, they were quarantined for some time afterward.

    On the topic of hand lasers- but the time we can travel to the stars, I expect current crude prototype laser weapons to be refined and mature weapons. Current lasers can blast through steel. Some can ionize the air to conduct an electrical charge that can stun or kill. Others use intense laser pulses to create plasma bursts that stimulate the nervous system, paralyzing or killing. I don’t think the hand laser is such a ridiculous weapon- what is holding it back is the lack of a portable power supply and the bulk and inefficiency of modern lasers.

    To Martin J Sallberg: you are right that space is full of resources. Asteroids are full of metals, comets provide volatiles, and gas giants can provide fuel for fusion reactors. Perhaps our descendants will live in vast ships that wander the cosmos, unfettered to any planet. They could mine asteroids and comets along the way. Energy would come from fusion reactors. If space drives are ever invented, they wouldn’t even have to worry about finding propellent for a rocket engine. We could try this lifestyle in our own solar system. Sort of reminds me of the SF novel Cities in Flight.

    I don’t think generation ship technology will be used only to colonize another planet- if you can create a closed life support system like that, why live on a planet? You can wander the cosmos, free from any planet’s grip.

  • Ronald October 21, 2011, 9:07

    Tony P and Paul: gravitomagnetism and its hypothetical counterpart, electrogravity, are not only postulated by Tajmar, but before him by the German physicists Dröscher and Häuser, that elaborated on Heim.
    Equally unconfirmed. But maybe future experiments using superconductivity can shed final light on the matter.

  • Eniac October 21, 2011, 12:37

    I wish to work on speculative propulsion physics- imagine how a propellentless space drive that created thrust anywhere in the universe would impact space travel!!

    A propellantless drive does indeed exist (http://en.wikipedia.org/wiki/Nuclear_photonic_rocket). It is very easy to engineer, you probably have one in a drawer somewhere, called “flashlight”. The problem with it is the amount of power needed for a given thrust, which is P = T*c. Unfortunately the laws of nature as we understand them dictate that this be so for any propellantless drive (Yes, our old friend Conservation of Momentum, which is deeply rooted in symmetry considerations and will not budge easily). It means that to be efficient as a rocket, such a drive will have to carry so much energy that the relativistic mass of the energy itself becomes subject to the rocket equation, and we are left where we were when we had propellant.

    Even so, one very attractive concept is the anti-matter photonic rocket. You get around the problem of directing the mesons and gamma rays by capturing them in a sufficiently thick shield. You use the thermal radiation emitted by the shield for propulsion, via a parabolic mirror. This scheme can be nearly 100% percent efficient, since all the energy liberated by the reaction ends out as directed thermal radiation. It also operates at the maximum rocket efficiency, with specific impulse of c and the entire fuel mass converted into propulsion energy.

    However, and there is the rub, if you calculate the radiation pressure for a black body at 4000K (about the maximum sustainable on a solid shield), you find that the thrust comes out at about 0.05 N/m^2, which is way too low given the thickness and density (mass per m^2) such a shield would need to have. I don’t know a way out of this, but that doesn’t mean there isn’t one…

  • Christopher Phoenix October 21, 2011, 17:25

    Hello, Eniac…

    I am well aware of the photon rocket, but it is just that, a rocket. You have to carry along nuclear fuel, which fissions, fusions, or undergoes annihilation reactions- and your specific impulse goes down as you factor in the mass of the reactor, parabolic shield, etc.

    I read Eugene Sanger’s original paper on the positron/electron rocket. He knew that our space efforts needed a much more powerful and efficient rocket than the chemical rockets used then and today if humanity was to reach for the stars. His designs called for an exotic technology called an Electron Gas Mirror. The positron/electron rocket spews out gamma rays, which need to be collimated into a nice output beam. No ordinary mirror can reflect gamma rays, so Eugene Sanger suggested a mirror made of free electrons- an electron gas. This structure would be unstable due to the negative charge of the electrons repelling each other, but several electron guns directed toward the back of the rocket might create such a mirror. The positron/electron annihilation rocket would reflect the gamma rays produced by the annihilation reaction with a parabolic electron gas mirror, creating thrust for a starship.

    An interesting thing to note is that the Electron Gas Mirror would also enable the weapon beam, or ray gun. A big problem with creating super-powered ray guns that can vaporize enemy spaceships is the mirrors needed to direct the beam- the mirrors will absorb some of the energy and crack. You need to keep the beam diffuse in your ray gun and cool the mirrors so your death ray generator does not melt itself to slag. An electron gas mirror could reflect high energy radiation like x-rays and gamma rays at 100% efficiency. These high energy rays are more penetrating and deadly than UV rays or visible light, but are somewhat hard to direct since they cannot be reflected.

    Alas, electron gas mirrors seem to be beyond our current technology. I have not even seen anyone trying to create one- which seems strange to me. I would expect the military to be interested in perfectly efficient mirrors that enable super-powered weapon beams. I think this is why DARPA is supporting the 100 Year Starship Project. So many spaceflight technologies have military spinoffs.

    However, the photon rocket is still a rocket. When I said “propellentless propulsion”, I really meant “reactionless drive”, “inertialess drive”, “space drive”, “gravity control drive”, “antigravity” and so on. Maybe I’ve read too much science fiction, but I still remember the Spindizzy from Cities in Flight. The Breakthrough Propulsion Physics Project looked into ideas like this under the names “differential sail”, “diode sail”, “induction sail”, “bias drive”, “diametric drive”, “disjunction drive”, and “induction ring”. And there is always the Alcubierre Warp Drive.


    The ever-useful Atomic Rockets web site mentioned these, concerning there use in SF, but made it clear that reactionless drives violate the law of Conservation of Momentum.


    I don’t see how fast travel between stars can ever be really practical as long as we have to carry around huge tanks of propellent. Rocket propelled starships will be a slow and tedious way to explore the cosmos. What I am looking for is some means of propelling a spacecraft without needed propellent anywhere in the cosmos, as in converting energy completely into forward motion- the hypothetical space drive. As the web site of the Breakthrough Propulsion Physics Program noted, if humans are ever going to fly the stars as quickly and easily as we have always dreamed, than we need propulsion systems that don’t need huge tanks of reaction mass.

    Otherwise- expect decade or even century long voyages on rocket propelled starships. Antimatter power and photon drives could allow us to travel to the stars, but such ships will be slower than we would like. Reaching high relativistic speeds is difficult because the mass ratio spirals into the impossible as you attempt to approach the speed of light, and then you need to slow down. Except you have to propel the propellent you need to slow down in the first place- so you need even more on the outgoing portion of the trip. Fast starships will be very difficult, if not impossible, without space drives. You end up with truly ugly mass ratios.

  • Ole Burde October 22, 2011, 5:15

    Whatever way of propulsion system will become possible , I think time has come to broaden this kind of discussion to include the gigantic problems related to MAINTAINING whatever starship kan be build for hundreds of years . This includes maintaning a biologic system , incuding the crew, who might proove to be the biggest of al chalenges .

  • Rick Robinson October 22, 2011, 12:09

    Many years ago I read a very simple and handy scale visualizer, that still strikes me as the best I’ve heard of – its only flaw being that it uses non-metric units.

    In any case, the number of AUs per light year is almost identical to the number of inches in a mile. So, sketch the Solar System on a scale of 1 AU = 1″, then walk briskly for an hour to reach Alpha Centauri.

  • Eniac October 22, 2011, 13:28

    @Chris: I think you may be missing my point that reaction mass is not really the problem. An efficient rocket uses its spent fuel as reaction mass. Thus, a reactionless drive does not really save much, because you still have to carry the fuel, for the energy. This gets worse in relativistic flight: The energy itself weighs enough to drag you down.

    Yes, a photonic rocket is still a rocket, but it does fullfill the one criterion you name: It does not require propellant. You are looking for a drive that needs neither propellant nor fuel, but then you have to conjure up both momentum AND energy out of nothing.

    So, even if we disregard the fact that a “reactionless” drive is completely unphysical, such a drive will not overcome the rocket equation. The only way to do that is an external supply of energy and momentum, but we do not know a plausible method for this that will allow you to stop at the destination. Landis’ multi-stage sail is a possible exception, but it really stretches the definition of plausible.

    I don’t know if you got this point about the photon rocket: It allows you to utilize gamma rays and most other reaction products without reflecting them. Unlike the “electron gas mirror”, it requires no near-magic technology except for the antimatter handling, which of course is rather prohibitive by itself.

    The fact is, though, for a “good” drive we need one that can turn 100% of fuel mass into energy, and the only ways we know to do that are antimatter and black holes. I am afraid we are stuck with fusion for a good while…

    Large mass ratios by themselves are not all that ugly if the fuel is solid, stable and reasonably affordable. You could then take huge amounts as a solid cone or a “train” of blocks without worrying about containment mass. Lithium deuteride is such a fuel, if we could just figure out how to burn it efficiently. Luckily, lithium deuteride is also the easiest to burn. Most H-bombs use it, and planned fusion reactors are fueled with lithium and deuterium.

    You are right that it would be nice if we had the miracle drive you speak of, but to get that we would first need some new laws of physics. We may get some of those, one day, but there is nothing but hope to tell us the new ones will be any more forgiving than the old…

  • Brett Bellmore October 22, 2011, 15:20

    Why not use a mass beam propulsion system? The concept I have in mind is to direct a beam of EM radiation in the direction of the starship. You then introduce small solar sails into the beam. They accelerate all the way to the starship, and then impact a pusher plate, probably a magnetic field rather than a solid plate.

    With this concept, the further the starship gets from it’s launch site, the faster the arriving mass beam is traveling. And the starship can provide a beacon for the individual solar sales to home in on, so the beam doesn’t have to be so tightly focused.

    The ship could probably capture some fraction of the mass beam to make up losses en route…

    The downside I see is that the sails would shadow each other. You’d have to keep the total mass in the beam down to prevent that. And, of course, a mass beam can’t slow you at the other end.

  • Christopher Phoenix October 22, 2011, 18:26

    @Eniac: I went back and reread the article on nuclear photonic rockets. You are right- the photon rocket wikipedia describes uses blackbody radiation from a very hot nuclear reactor to provide thrust. There is not separate tank of reaction mass, only nuclear fuel. Nor is there an electron gas mirror. The photon rocket simply radiates away EM radiation from photon radiators that are heated by the reactor. You are also right that eliminating propellent will not defeat the rocket equation.

    I have a question- you stated that a photon rocket can make use of the reaction products, including gamma rays, without needing to reflect them. From what I’ve read, a photon rocket would radiate EM radiation away from a very hot graphite or tungsten photon radiator. Wouldn’t a lot of the heat simply escape in a direction that did not contribute to thrusting our rocket or just conduct away into the radiator’s supports? It seems to me that reflecting the radiation would be more efficient, as the Wikipedia article suggests.

    In fact, the Wikipedia article stated: The power per thrust required for a perfectly collimated output beam is 300 MW/N (half this if it can be reflected off the craft); very high energy density power sources would be required to provide reasonable thrust without unreasonable weight.

    It seems to me that miracle technology is needed to create a interstellar-worthy photon rocket. We’d need some way to convert matter completely into energy, like antimatter, black holes, or some new exotic nuclear decay. And a means of reflecting gamma rays would make the rocket twice as efficient. Eugene Sanger’s original concept was to produce gamma rays by allowing positrons and electrons to collide and then reflect them into a collimated output beam with a perfectly reflective “electron gas mirror” that sounds very similar to a SF forcefield. He also wrote of rockets with fusion reactors- hot balls of plasma that radiated energy that reflected off the mirrors. All of his designs included mirrors.

    You are right that eliminating propellent does not free you from the rocket equation unless you have an external source of energy and propellent, at least with current physics. My mistake- NASA’s Breakthrough Propulsion Physics Program web site mainly discussed the propellent issue. However, they neglected to mention the fact that a thruster that could propel a spaceship with greater performance than a photon thruster (i.e carrying around the energy needed to power it did not slow you down) breaks the entire mathematical framework of physics, violating not only Conservation of Momentum but Conservation of Energy. There might be a way out, but it will be exotic new physics that provides it.

    I’d be very happy if fusion propulsion became standard on human spaceships- we could do it now if we used a nuclear pulse drive. The whole solar system would open to exploration or even colonization.

    On the large mass ratios- I’ve heard that it is probably impossible to build a single stage rocket ship with a mass ratio over 20, due to something called “minimum gauge”. Basically, the parts of your rocket- the fuel tanks, pipes, support struts, and combustion chambers have to deal with certain forces and pressures that don’t get smaller just because you want a smaller rocket. The pipes need to be thick enough to withstand certain pressures. The support struts need to be strong enough. The combustion chambers have to be rugged. Thus, there is only so much mass you can shave off a rocket before the fuel tanks pop like balloons and the support struts snap like toothpicks. So you need to either stage a rocket or build a HUGE rocket ship if you want high mass ratios.

    Does this mean a rocket starship (I prefer the term “light-year rocket” from Ray Bradbury’s short stories) would have to be very huge or perhaps even staged?

    If you can store your fuel in solid form without needing a tank, could a rocket simply carry a big block of propellent? In that case, perhaps a big mass ratio is not so terrible.

    The Millis space drives utilized such bizarre concepts as vacuum energy pushing on a sail, negative mass, or theoretical connections between electromagnetism and gravity. These drives are not rockets, and cannot be modeled with the rocket equation. Some people tried to model them by imagining them as rockets with infinite specific impulse, but that just does not work.

    It would be wonderful if we had exotic propulsion like this. Someday I will look into it. Quantum physics tells us space is not empty- it is seething with virtual particles that pop in and out of existence, violating conservation of energy but disappearing before we can directly measure them doing so. We can detect the forces of the quantum vacuum- it is called the Casimr force. Could a “Quantum Drive” use the vacuum fluctuations to propel a spaceship? Perhaps, but we’ll need to investigate new physics to find out.

    The basic question is, if we changed space-time around a craft, would space-time then push on the spacecraft (or flying saucer!!) and move it? This is all very speculative and attracts the lunatic fringe. No breakthroughs seem imminent, but breakthroughs do tend to take us by surprise. If we can’t build induction rings or warp drives, than we are stuck with rockets, so we better study up on rocketry.

    And, as you pointed out, there is no guarantee that a new physical theory will be any kinder to us space cadets than the old ones are!!!

  • Eniac October 22, 2011, 22:15


    I have a question- you stated that a photon rocket can make use of the reaction products, including gamma rays, without needing to reflect them. From what I’ve read, a photon rocket would radiate EM radiation away from a very hot graphite or tungsten photon radiator. Wouldn’t a lot of the heat simply escape in a direction that did not contribute to thrusting our rocket or just conduct away into the radiator’s supports? It seems to me that reflecting the radiation would be more efficient, as the Wikipedia article suggests.

    You would still be reflecting the exhaust, but they would not be gamma rays and mesons, they would be mostly visible light and thus easy to reflect. With a good parabolic mirror (or better: light sail), you could achieve a degree of collimation that would have better than 90% propulsion efficiency (<10% of exhaust getting away in directions other than straight behind). The beauty is that you need not worry about thermal losses, because essentially you are turning those into a virtue by utilizing thermal black-body radiation for propulsion. It is much easier to design a reactor with 100% thermal "loss", than one that efficiently produces electricity or a high energy exhaust plume. It is sad that the thrust/weight ratio issue makes this impractical, but this limitation may be subject to engineering solutions, one does not need new laws of physics.

    If you can store your fuel in solid form without needing a tank, could a rocket simply carry a big block of propellent? In that case, perhaps a big mass ratio is not so terrible.

    Yes, this was exactly my point. You could have arbitrarily large mass ratios, but keep in mind that the delta-v returns diminish quickly because of the exponential nature of the rocket equation. The most natural shape for a high mass ratio rocket is some sort of cone, with an exponentially increasing cross section from the tip to the base. The cone will be 100% fuel, and the base is covered with engines, which will be gradually discarded as the fuel is used up and the base cross-section becomes smaller. You could call each generation of engines a “stage”, except you would use some of the engines from the previous stage for the next, making the system a little more efficient than true multi-stage where you throw away the entire engine.

    It is possible that the “aerodynamics” of the ISM will force us to keep a minimal cross-section to minimize impact radiation damage. Then, the optimal shape is a very long rod, or a train of individual fuel blocks. Engines, in this case, would have to be at the front, sacrificing some propulsion efficiency as exhaust must be directed at an angle away from the train.

    The Millis space drives utilized such bizarre concepts as vacuum energy pushing on a sail, negative mass, or theoretical connections between electromagnetism and gravity.

    Usable vacuum energy is a contradiction in terms. “Vacuum” is defined as the lowest energy state, and using it implies a lower state, contradicting the definition. And without “free energy”, the rocket equation still applies, no matter how exotic and propellantless the propulsion.

    Space-warps are not really propulsion. They could offer elegant solutions (like star gates), but without a plausible way to actually effect the warping of spacetime, they will remain thought experiments.

  • Christopher Phoenix October 23, 2011, 19:16


    “You would still be reflecting the exhaust, but they would not be gamma rays and mesons, they would be mostly visible light and thus easy to reflect.”

    I see your idea- you want to heat up a photon radiator with a nuclear reactor. This photon radiator will radiate visible light we can reflect of a light-sail. However, the solid shield can’t withstand high enough temperatures to obtain significant thrust. Maybe the shield does not have to be solid. Could it be a dense plasma trapped in a magnetic field? Like a small sun, such a dense plasma would radiate light to reflect off of our photon ships reflectors. Maybe a solution will be found someday. On the plus side, we don’t need any new physics, we just need an engineering breakthrough.

    Sort of like the gas-core nuclear rocket. The solid core nuclear rocket is limited by the temperature the solid reactor can attain without being melted. However, in the gas core nuclear rocket, a U-235 plasma rotates in a magnetic field, radiating heat to boil hydrogen propellent. The gas core can achieve much higher temperatures than the solid core since it is already gaseous. The Discovery from 2001: A Space Odyssey used a gas core reactor to produce electricity to drive plasma rockets.

    I have my doubts that an antimatter reaction could power such a rocket with near 100% efficiency. Antimatter reactions are messy, in the case of protons and anti-protons. Some energy will escape in the form of neutrinos.

    Eugene Sanger originally conceived of the photon rocket, suggesting a design where electrons and positrons collide to produce high energy gamma rays. The gamma rays were the source of the thrust. The problem (other than storing the antimatter) was that gamma rays are impossible to reflect. Eugene Sanger tried to design an “electron gas mirror” for this purpose, but failed. Thus, the positron/electron rocket remains science fiction.

    Nowadays, rocket engineers have abandoned Sanger’s original design for an antimatter rocket, since there is no way to utilize the high energy gamma rays and neutrinos from the antimatter reaction. The more likely design for an antimatter rocket is the “pion rocket” or “beam core antimatter rocket”, which directs charged pions from the collision of hydrogen and anti-hydrogen out the back of the rocket with a magnetic nozzle. The electrons and positrons would be stripped of the hydrogen and anti-hydrogen to avoid producing deadly, useless gamma rays. The charged pions are utilized for thrust. The performance of such a rocket is less than the ideal gamma-ray “photon rocket”, but still impressive.

    Don’t take my word for it. Check out these links.

    James Pellegrino’s Valkyrie Starship utilizes a beam-core antimatter rocket as well, bouncing pi-mesons produced in a proton/anti-proton reaction off of a magnetic field to produce thrust. The Valkyrie also turns conventional rocket engineering on its head, putting the engine in the front of the ship and pulling the crew module behind on a long tether to shave off structural mass.

    On the fuel blocks- intriguing idea, but it does ruin the sleek lines of a rocket ship. Small price to pay for actual star travel. I have another nit-pick, however. You mentioned the fuel should be not only stable and solid, but affordable. Where are you going the get the lithium deuteride? Is it affordable enough? Lithium is a light atom, so it should be relatively common. What about deuterium? There is only one atom of heavy water for every 6,420 of ordinary hydrogen in Earth’s oceans. Deuterium is destroyed faster than it is created in stars, so I would worry that it is not common in the universe. According to wikipedia, Jupiter and the comets have a higher concentration of deuterium. Still, having to mine comets and gas giants to obtain thousands of tons of fuel for a spaceship will be expensive.

    The interstellar medium will probably force us to make our ships sleek and needle-shaped. Interstellar shielding is an important consideration. We don’t want to be fried by induced cosmic rays. Another worry is debris, like icy bodies in interstellar space. Such a large object would destroy our craft. In one Arthur C. Clarke story, an STL interstellar spacecraft (driven by a Quantum Drive that utilizes the vacuum forces!!) has a large block of ice on the front to shield the crew from the induced cosmic rays. The crew is in suspended animation for the duration of the voyage.

    “Usable vacuum energy is a contradiction in terms. “Vacuum” is defined as the lowest energy state, and using it implies a lower state, contradicting the definition. And without “free energy”, the rocket equation still applies, no matter how exotic and propellantless the propulsion.”

    I think you missed my point. I wasn’t suggesting that we can mine the vacuum for usable energy. I was pointing out that the the “vacuum” is not really empty, and that virtual particles exert forces. If we can change the vacuum energy by manipulating the Casimr forces, we might be able to manipulate space-time to create exotic propulsion systems.

    The Differential Sail, a hypothetical space drive, is based on speculation that that it might be possible to induce differences in the pressure of vacuum fluctuations on either side of sail-like structure — with the pressure being somehow reduced on the forward surface of the sail, but pushing as normal on the aft surface — and thus propel a vehicle forward. Speculation, yes, but still interesting. Other ideas suggest that by altering the gravitational constant around a spacecraft, a gravity field could be created that would propel the craft.

    It is somewhat like that experiment you might have done as a young child- placing a little aluminum foil and cardboard “boat” in a basin of water and dropping detergent behind it. The boat shoots off away from the detergent, even though it has no propellers or sails. What you did was change the water around the boat, which in turn moved the craft. The detergent changed the surface tension of the water behind the boat, so there is an area of high surface tension in front of the boat and low surface tension behind it. The boat is pulled toward the area of high surface tension, just as the hypothetical space drive is moved by changes in the space around it. This is just an analogy, of course. Whether we can change space-time like we can change the surface tension of water remain to be seen.

    As for the rocket equation applying to hypothetical propulsion techniques that use only interactions between the ship and surrounding space- you can take that up with Mark Millis. You can’t use the metrics of rocketry to describe hypothetical space drives.
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070031912_2007031208.pdf (skip to Avoiding Pitfalls)

    The flashlight, I mean photon rocket, can propel a craft without having to carry along a separate tank of propellent- although the nuclear fuel needed to produce the electromagnetic radiation to propel the ship would weigh you down. The light the ship directs out of the engine does not have mass, even though it carries momentum and exerts pressure. The ship is still a rocket. You have not defeated the rocket equation. As your ship thrusts, it converts mass to energy and uses up its fuel, becoming lighter. It is an intriguing idea, but I doubt we can build a starship worthy photon engine any time soon.

    To escape the rocket equation with current physics, we will need a separate source of energy and mass- like the laser propelled light-sail or externally fueled fusion rocket. The Bussard Ramjet is a neat idea, but seems impractical. Most of the ideas that do this, like beam-riders, don’t have any easy way to slow down. The laser light-sail does, but it stretches the bounds of credibility.

    The only way to really escape the rocket equation will be to find new physics. Physicists who explore ideas like warping space, creating wormholes, or finding some means to alter space around a craft to propel it are speculating. When we discuss credible starship design with current physics, we are stuck with rockets or beam-riders. My point remains, though- the big problem is carrying along all that mass and energy we need to fuel our ship.

  • Tobias Holbrook October 24, 2011, 8:34

    You can go twice as fast if you use beamed energy to speed up and use your propellent to slow down. You can go even faster if you can use a magnetic sail in the deceleration phase…

  • Eniac October 24, 2011, 8:54

    Chris: 1 out of 6420 in seawater still is a whole lot of deuterium, if you ask me. Likewise, Wikipedia says there are 230 billion tons of lithium in seawater, probably easier to reach than the outer planets.

    “Differential sail” is just another perpetual motion machine, might as well design a terrestrial sail that lets air molecules through in only one direction. Aka Maxwell’s daemon, and forbidden by the second law of thermodynamics. Since energy is mass, it is quite clear: either you create energy out of nothing, or you take it from somewhere en route (but not the vacuum, please), or you store it on-board and abide by the rocket equation.

    I like your soap boat analogy. You could speculate about something like that, but it would be quite dangerous: The soap thing works only once on the same water, and we can’t be sure a universe without surface tension would still allow us to exist….

    You would not want to use ice for shielding, it would ruin your mass ratio. You would want to use fuel. At least if you can ensure it won’t detonate. Word on the street is lithium deuteride cannot detonate, but I wonder sometimes if that is just misinformation to keep someone from trying to set off a really large bomb. A truck full of detonating lithium deuteride could really ruin your day.

  • Eniac October 24, 2011, 9:16


    Why not use a mass beam propulsion system? The concept I have in mind is to direct a beam of EM radiation in the direction of the starship. You then introduce small solar sails into the beam. They accelerate all the way to the starship, and then impact a pusher plate, probably a magnetic field rather than a solid plate.

    In my opinion, this is the best of the beam ideas. You may want to check out this paper, although it seems like you already did:


  • Christopher Phoenix October 24, 2011, 19:36


    True- the differential sail would have the effect of cooling space as it accelerated. Thus, it violates the 2nd Law of Thermodynamics. Good call!! Anyway, I can now explain why the differential sail will not work.

    Take the example of a sail designed to use the cosmic background radiation. Space is full of microwave radiation, very close to being isotropic in all directions and essentially thermal blackbody radiation with a temperature of 2.7 Kelvin. We now imagine a large structure, or sail, that perfectly reflects radiation on one side and completely absorbs radiation on the other side. Since light exerts twice as much pressure on a reflecting surface as an absorbing one, we conclude that the sail will experience a net force and accelerate. However, the sail will heat up, since it is absorbing radiation on one side. The sail will become a blackbody emitter itself. That radiation carries momentum and exerts a force on the sail, like a rocket. Soon, the differential sail will reach thermodynamic equilibrium and will be the same temperature as the cosmic background radiation. The amount of light leaving the sail on the absorbing side is equal to the amount bouncing off of the reflective side, and the momentum change is no different for the absorbing side than the reflecting side. There is no net force on the sail and it provides no more thrust.

    And this all assumes the sail started out at 0 degrees Kelvin!! In reality, the sail will start out at the same temperature as the background radiation unless we cool it down in a refrigerator. If we did cool it down, than it only provides thrust until it warms back up again. Also, where do you find a perfectly reflective and absorbing material? This all seems contrived. The other sail concept, the diode sail, won’t work either, since there is no material that allows radiation through one way but not the other.

    Not that I am weeping for the differential sail- I never liked it much anyway. I had the feeling that something was wrong with it, I just hadn’t figured out what. It would be the slowest space drive imaginable, anyway. I would much prefer the Induction Ring.

    I think it is a little odd that a NASA web site discussed the differential sail and diode sail without discussing the fact that they violate the Laws of Thermodynamics. They did say that all these devices were purely hypothetical, but still… I would have expected to find them in the “Ideas that don’t work” section.

    What I am really after is not violating the Conservation of Momentum or the 2nd Law of Thermodynamics, but some method of transporting as spaceship at extremely high speeds to the stars without needing to carry along a massive amount of fuel or propellent. This requires new physical laws that may or may not exist, but it is worth looking. Even if we find it is not possible, we will learn something about physics along the way. Space drives might not require us to break the laws of physics if there is some way to alter space and cause it to drag us along- that is not really propulsion, but we are still getting where we want to go. Effective FTL drives might not require FTL motion if we can warp space enough to contract space or create warp drives. The main problem with FTL drives is that they allow us to create intricate situations where causality is violated- effect precedes cause.

    As for the soap boat trick working only once- I’m hoping that is where the analogy between space drives and soap boats breaks down. Let’s hope I don’t destroy the livable space around Earth by accident by creating the space-time equivalent of a soap boat in my laboratory! I’m crazy enough to try something like that…

    The ship that used an ice shield had a Quantum Drive, so it did not have to worry about mass ratios. Fuel as a ship’s shield? Maybe… but I would’t like the fact that you were using up your shield as you went along. Sort of like those Mars ships that use the ships propellent as shielding material against solar storms. Your protection goes away as you use up the propellent.

    Maybe there is enough deuterium and lithium in the oceans, but you still have to mine it. What about launching all that fuel into space? Fuel is a pain in the neck. Let’s hope we can find some means to turn matter totally into energy someday, like Hawking black holes. Then we could refuel from any source of matter.

    Any sufficiently powerful reaction engine could be used as a weapon, you know. The exhaust from a fusion rocket could melt any nearby object. A photon engine would be basically a giant laser, capable of cutting another ship in half from close range. A bank of laser cannons based on the moon to accelerate a light-sail could also be used as a deadly death ray weapon. The massive kinetic energy of a starship moving at a significant fraction of light speed would make it a deadly weapon. Just google “relativistic kill vehicle”. Or read “The Killing Star”.

    Of course, the exhaust of most engines would be poorly collimated and only effective at short range. There are always the fusion pumped X-ray pulse laser turrets. Or the nuclear-tipped guided missiles. Or the mass drivers. Or the Lunar Phased Array Laser… As Arthur C. Clarke said, nuclear power makes space travel not only possible, but imperative.

    I think this is part of the problem with a Orion-style nuclear pulse drive with clean bombs instead of fission bombs- if it is possible to set off fusion bombs without a fission trigger, than terrorists and small countries will be able to assemble nuclear weapons in secret. Nuclear weapons are controlled by tracking fissionable material. If it is possible to trigger fusion bombs without a fission trigger, someone could set one off in a major city. Maybe even build a neutron bomb. The population of a city could be left lethally poisoned, retching their guts out to die a horrible death a short while later. Death from neutron bombs comes slowly. I’ve even wondered if some physicist discovered the secret to detonating “clean bombs” but hid it to avoid the consequences of fusion bombs that cannot be tracked. That secret could allow us to build a fleet of nuclear pulse spaceships to travel through our solar system. A safer plan is to detonate pellets of nuclear fuel with a laser in the engine.

    On a side note, a tiny amount of antimatter can trigger a clean bomb. Unfortunately (or fortunately), antimatter is very expensive. The future nuclear arsenal could be based on antimatter triggers. Photon torpedoes are a lot farther away.

  • Eniac October 25, 2011, 9:31

    The massive kinetic energy of a starship moving at a significant fraction of light speed would make it a deadly weapon. Just google “relativistic kill vehicle”. Or read “The Killing Star”.

    This one is not a problem, as a relativistic starship could only threaten other star systems, not Earth. By the time it achieves relativistic velocity, it is safely far away and definitely not coming back.

  • A. A. Jackson October 25, 2011, 10:15

    Christopher Phoenix
    “Maybe I’ve read too much science fiction, but I still remember the Spindizzy from Cities in Flight. ”

    Lord! James Blish trumped all SF writers with CITIES IN FLIGHT.
    He not only invented the FTL ‘spindizzy’ (actually The Dillon-Wagoner Graviton Polarity Generator, what a name, Blish must of had a lot of fun!)
    Blish doubled the bid! Not only FTL drives , but also Anti-agathic drugs for near immortality!
    None of the great science fiction writers thought small but Blish was in class by himself. (Alas, like Sturgeon, Phol, Kornbluth, Cordwainer Smith, many many others, not remembered as being the equal and many times superior to to the big three.)
    In CITIES IN FLIGHT one just ripped out a whole city, after all all the infrastructure is there! Super science supported its logistics and life support. Surrounded by a force field New York city (others too) speed about the universe at FTL speeds while the inhabitants lived forever. Sounds kind of silly, no? Well no, Blish constructed intelligent and sophisticated and very engaging , beyond space opera stories in the four novel Okie series.

    For us Tau Zeroians (don’t want to say Tau Zeros!) , Blish beat Poul Anderson by about 11 years with The Triumph of Time , the last Okie novel in 1959, with the universe coming to an end and the creation of another universe by our heros! (Blish didn’t quite beat multi-universe physics, but got there years before it was a hot topic.)

  • ljk October 25, 2011, 12:08

    Eniac said on October 25, 2011 at 9:31:

    The massive kinetic energy of a starship moving at a significant fraction of light speed would make it a deadly weapon. Just google “relativistic kill vehicle”. Or read “The Killing Star”.

    “This one is not a problem, as a relativistic starship could only threaten other star systems, not Earth. By the time it achieves relativistic velocity, it is safely far away and definitely not coming back.” [end quote]

    Are you assuming humanity would launch starships at other systems as a weapon? Then I guess we better hope that the target system is completely wiped out and/or any survivors are not big on revenge, or that there are not sufficiently advanced galactic societies that frown on such behavior and take steps to stop such cosmically aggressive species.

  • Eniac October 25, 2011, 19:57

    Are you assuming humanity would launch starships at other systems as a weapon?

    No, not at all. I was just pointing out that relativistic kill vehicles cannot possibly be used by Earthlings against Earth, making them a non-issue regarding the question of whether we should develop a starship.

  • Christopher Phoenix October 26, 2011, 0:48

    @ A.A. Jackson:

    I loved the Spindizzy!! James Blish even gave us an equation for it. He based it off of the work of the physicist P.M.S Blackett, who tried to correlate the known magnetic fields of large rotating bodies, such as the Earth, the Sun, and a star in Cygnus who’s rotation had been measured indirectly. This equation brought Isaac Newton’s gravitational constant and Coulombs’ constant together. These equations were later disproved by more accurate measurements and the discovery of magnetic field reversals on the Sun and Earth.

    James Blish’s extrapolation was that if rotation+mass produces magnetism via gravity, then rotation+magnetism could produce anti-gravity. The field created by a spindizzy is described as altering the magnetic moment of any atom in its influence. This drive became more efficient when it lifted greater masses, and this was the hook for the story- it was more effective to lift entire cities than something smaller, like a traditional spaceship. Space vehicles did not need to be made small and lightweight. You could build a spherical hull, fill it with necessary equipment, and then put in a spindizzy and take off. Entire cities could convert themselves to spaceships and wander the galaxy at FTL speeds.

    And don’t forget the bridge on Jupiter!! When I first read “They Shall Have Stars”, I was skeptical. Move an asteroid? Drop it into Jupiter’s atmosphere and use it as a stable foundation for building a bridge out of ice? And this is the achievement of a space program that is stagnating, no longer developing any new technology? This all seemed rather fantastic.

    The anti-agathic drugs were very interesting. Come to think of it, if we had those today, the present generation could live to the age of star travel.

  • Christopher Phoenix October 26, 2011, 0:55


    I was pointing out that with the energy content of a reaction drive that could propel a starship at any decent speed it would be a deadly weapon, if its creators were less than benevolent. Imagine if a terrorist got hold of such a craft. If you don’t comply with our demands, we will drop a portion of our antimatter fuel supply on populated areas…

    Any spaceship travels at pretty high speed, making kinetic weapons fairly deadly. We might stop a marauding alien rocket ship making diving toward Earth with the intention of bombing our cities with nothing more than a simple lump of meteoric iron tossed in its path. Rocket ships do move fast…

    However, as you pointed out, using kinetic kill vehicles- relativistic or otherwise- is hard to do without a large area of space to accelerate in. True, you could bombard a distant space station or planet with a lump of iron strapped to a rocket, but you can’t use that at very close range. At close range, we are stuck with coil-guns, lasers, etc. It is impossible to use such a kill vehicle against nearby targets or the Earth.

    The launch of a relativistic missile would be a premeditated, deliberately aggressive act by a civilization. Launching this missile would require a large amount of energy and resources. Using such a weapon would indicate to any nearby civilization that you are hostile and also reveal your location. Best not to use such a weapon- if there are any nearby sentient species, they might react to our hostility by launching their own weapon. Any survivors of the attacked civilization would be bent on revenge. It would be hard to track down and destroy an entire species if they scattered in hollowed out asteroids and comets, space stations, and starships. It is also possible that we are not the only kids on the block- a sufficiently advanced galactic society might take steps to stop a cosmically aggressive species. It is much better to make an impression as peaceful explorers.

    For those close range engagements where kinetic kill vehicles are not practical, I prefer the Mark IV Hawk combat spacecraft. Fast and agile, the Mark IV Hawk is essentially a two-man cockpit mounted on a single fusion engine and fuel tank. She can accelerate at up to 20g and is completely modular. (The Hawk is from the Space: 1999 universe and she has antigravity screens, so her crew not only is not crushed by 20g’s of force.)

    Dual weapons pods house computer targeted laser batteries, 16 independently activated missile launchers, and two self-guiding torpedoes with fusion warheads. An additional laser battery is located on the underneath of the forward Command Pod. All weapons are operated manually by a gunner or programmed to follow a predetermined attack plan. In case of emergency, the propulsion and weapons sections can be jettisoned and the Command Pod can be used as a lifeboat.

    Despite the absurd premise of Space: 1999, I like the spacecraft design of several craft from that series. Most media SF has ships that look like aircraft or behave like boats. The modular design and presence of rocket engines gave an air of plausibility too several of the Space 1999 ships.


    I wonder if the Mark IV Hawk used lithium deuteride as fuel… I guess the question of whether lithium deuteride could detonate would be really important to a Hawk crew, if an hostile weapons fire hit the fuel tank!!

  • Eniac October 26, 2011, 11:11

    Galactic society? There can be no such thing, given of the enormous communication lag between star systems. It pretty much completely prohibits negotiation or coordination of any sort. No governments, treaties or trade agreements could deal with interstellar distances. Not warfare, either, although you could argue that a malicious, unprovoked unilateral strike is possible, if certainly not reasonable. You would be killing people many, many years in the future without knowing a single bit about what they are up to now, much less then.

  • ljk October 26, 2011, 15:23

    Eniac said on October 25, 2011 at 19:57:

    Are you assuming humanity would launch starships at other systems as a weapon?

    No, not at all. I was just pointing out that relativistic kill vehicles cannot possibly be used by Earthlings against Earth, making them a non-issue regarding the question of whether we should develop a starship.
    [end quote]

    That is right – all any Earthling needs to wipe out someone else on Earth is the right kind of planetoid, a sufficiently efficient rocket engine, and a guidance computer.

  • Christopher Phoenix October 26, 2011, 18:35


    I should have said “interstellar society”, not “galactic society”- galaxies are just too big!! However, within 30.7 parsecs (or 100 light-years) there are 512 spectral type G stars. These stars could possibly have Earth like planets in orbit around them. About 64 are located within 15.3 parsecs (or 50 lys) of Earth. You don’t have to range too far to have a lot of exotic destinations and real estate. In fact, the rest of the universe- murky images of infant galaxies and all- is likely just a mute backdrop to whatever interstellar exploits we will be up to in the centuries to come.

    You can’t be sure that an interstellar civilization won’t exist someday. You have to abandon your cultural preconceptions. Modern day society only focuses on short term prospects. Most companies won’t invest on a proposal that has a payback a few years down the line, and politicians have an even shorter attention span- one that only extends to their next attempt at reelection. This is partly why we have not gotten very far in space- no government official is willing to support a long range effort that will have a massive payback twenty years down the line- a payback that he will not see personally, since he has to cut back funds for unnecessary projects in order to garner support for his reelection, and if he is reelected, he has pet projects he cares more about than silly spaceships. Any society capable of launching an interstellar mission will be unlike any seen today. The star travel effort would have to be impervious to weekly popularity polls or quirky political systems that elect a new government every four years. In all probability, this effort would not organized by a government at all.

    I could imagine interstellar trade occurring someday, given sufficiently advanced technology. The cost of interstellar transport would have to be affordable, which seems unlikely today. However, given advances in technology, the harnessing of extraterrestrial resources, and the ability to generate vast amount of energy, it might be possible in the future. A Kardashev Type-2 society could have developed fast, relatively affordable star travel. It will require a massive step forward in the speed that we travel, but that is what Centauri Dreams is all about. It was not so long ago that people thought traveling faster than a horse can run would be lethal…

    What would this society trade? Not raw materials, plenty of those exist in the universe. Not manufactured goods, either- it is almost certainly easier to manufacture them where you are then fly them in from Alpha Centauri. However, an future society might be dependent on exotic resources that are very rare, like magnetic monopoles or hawking black holes. Magnetic monopoles are good for building compact motors and generators, super-strong materials (ringworlds!!), high energy density power sources (good for blasters and phasers?), impenetrable armor plating (general products hulls!!), monopole catalyzed fusion, and ramscoop fields (star drives!). If monopoles are rare, then perhaps they will be prospected for and traded. Then again, perhaps we will find out how to manufacture them ourselves. Another possibility is a frivolous trade in luxury goods like fine wines, antiques, original Rembrandt’s, etc. Imagine what a figurine carved from gourd-tree material by a pre-industrial alien civilization on Darwin IV would sell for.

    The cheaper and more affordable star travel becomes, the more reasons a human can think of to go. Exploration, colonization, trade, or even the tourist impulse could be motivation enough to a Type-2 individual. Imagine large star-carriers able to transport craft from one star system to another. Or interstellar cyclers. If contact is common between settled star systems, then some sort of society could very well arise. A common means of communication would be very helpful. Star systems would not be isolated islands if star-carriers dropped in every few years or so. Even if this means decade long journeys, would that necessarily be an obstacle? Any practical star drive would propel the star-carrier to high speeds. The travelers would experience a shorter journey due to time dilation. Far future humans will probably live longer. Techniques are being researched today that can stop aging and extend life. Anti-agathic drugs could become a reality. In this case, the long travel times are not really much of a problem.

    As for propulsion- perhaps these ships have almost 100% efficient mass-to-energy conversion engines fed by a ramscoop. The dream of unlimited energy and propellent for a spacecraft is hard to let go. True, we can’t build one now, but it is more plausible than the warp drive from Star Trek. In a setting like the one I describe, there would be a loose interstellar society of travelers, traders, habitats, and planets.

    You could argue, even given my example, that different planets would be separated too much to have a shared society- but I don’t expect a shared society. In a STL setting, you can’t know what a colony twenty light years away is doing right now, but some traveler might tell you what it was like thirty years ago. When Marco Polo went to Cathay, he was away for 24 years. Perhaps future interstellar destinations will be like the indies, exotic, faraway, and offering strange experiences and perhaps wealth to those who visit.

    As for alien societies- how do we know what they are up to? They could live much longer than humans. They might have much more advanced technology. They might not like having a “Star War” going on down the interstellar street from them. Anyone who throws about relativistic missiles is a threat to anyone nearby, and they are obviously not friendly. In fact, any civilization that launches such an attack has proved themselves to be both malicious and unreasonable.

    If any sort of FTL hyperdrive or instantaneous transmitter exists, it would change all our conceptions about interstellar society. If starships could travel 100x faster than light, Alpha Centauri would be only about 19 days away. A journey to the nearest star with known planets, Epsilon Eridani, would take only a little over one month. Of course, such a drive would be based on as of yet undiscovered physics- and the biggest problem with FTL drives, other than the fact the present schemes would require incredible amounts of energy, is that they can create causality paradoxes.

  • Christopher Phoenix October 26, 2011, 20:26


    “That is right – all any Earthling needs to wipe out someone else on Earth is the right kind of planetoid, a sufficiently efficient rocket engine, and a guidance computer.”

    Any Earthling on Earth would just use a plain old nuclear missile, launched from a hidden silo or submarine. Why bother with space rocks when missiles are cheaper, quite destructive, and hit within minutes?

    Mass drivers and rockets can alter the orbit of an asteroid, which will be popular with asteroid miners who want to shift their claims into a different orbit. This will be unpopular with the astromilitary, which don’t think that civilization destroying asteroid bombardment is a power you want rock-rats to have.

    This actually is a good argument for a nation to want an fleet of combat spaceships, which makes it a nice concept for SF authors. The threat of any maniac space-man having the ability to launch an asteroid bombardment would give governments a good reason to invest in the development of space warships and military bases in space. A branch of the astromilitary will then patrol the solar system, watching for any unauthorized changes in an asteroid’s or planetoid’s orbit. If an asteroid starts to move without a permit, or strays off course, the military spacecraft of various nations will pounce and blow the snot out of it. All in self-defense, of course.

    This presents a somewhat more exciting future history than the peaceful, slightly-boring one that most space advocates predict. Once launch technologies can put massive payloads in space and self contained, long enduring habitat system’s are developed, the exploration and exploitation of space begins. Moon bases are built, missions are sent to Mars, and some companies begin sending rockets to near earth asteroids to mine their resources. High specific impulse, low thrust electric drives or solar sails can take ships out to the asteroids. Valuable metals like iron, titanium, iridium, and even gold and platinum will be sent back home, possibly in unmanned craft. Eventually, the miners will begin altering the orbit of asteroids to bring their claims into more useful orbits.

    This, however, makes many people nervous. Astromilitaries are formed to guard Earth from these potential impactors. A fleet of combat spaceships are built. Military moon bases,asteroid bases, or space stations house these ships. Civilian settlements spring up around military space bases. Some warships watch the asteroids, ready to sound the alarm and bring a punitive fleet down on any rogue planetoid. Military spaceships need both plenty of delta-v to get from place to place and the lots of thrust to maneuver. At first, dual drives might be used- say a ship with an ion drive to get from place to place and a chemical rocket to provide bursts of thrust.

    It is in the interest of astromilitaries to develop more powerful rocket engines. Combat spaceships need to be able to travel to an area of trouble swiftly and maneuver quickly in battle. Some kind of nuclear drive- like a gas core rocket, nuclear pulse rocket, or nuclear salt-water rocket is built. Eventually fusion rockets are built. This provides the first steps toward torch-ships (drives with both extremely high thrust and extremely high specific impulse). These new rockets allow spacecraft to reach destinations faster.

    These ships carry various kinds of kinetic weapons, missiles, and eventually laser beam weapons. Nuclear weapons are also an option, especially the ones like Casaba Howitzer that focus their blast in one direction. Flack cannons and fragmentation missiles are pretty deadly. A major concern is debris from space battles- when you blast an enemy ship with your cannons and nuclear missiles, it doesn’t just go away. The blasted hulk or debris will hurtle through space, possibly colliding with space traffic. The space fleets will need some way to clean up after themselves, like a laser broom or space tugboat.

  • ljk October 27, 2011, 9:50

    Christopher Phoenix said on October 26, 2011 at 20:26:


    “That is right – all any Earthling needs to wipe out someone else on Earth is the right kind of planetoid, a sufficiently efficient rocket engine, and a guidance computer.”

    Any Earthling on Earth would just use a plain old nuclear missile, launched from a hidden silo or submarine. Why bother with space rocks when missiles are cheaper, quite destructive, and hit within minutes? [end quote]

    Yes, nuclear weapons via missile (or suitcase or barge) are much cheaper and easier to use, but also a lot more obvious and relatively easy to locate and ban. Well, the nukes atop missiles anyway.

    But for a spacefaring nation or an interplanetary society/corporation, planetoids offer a “cover” of sorts while providing a devastating weapon that could level an entire nation in one shot and not even leave any messy radiation afterwards. The group that owns and occupies various planetoids can publicly claim they are there for scientific research purposes or mining the space rocks solely to create their interplanetary society infrastructure or for selling said resources to others for similar purposes. This has been done for decades with various space projects and such things as terrestrial polar “research”. Not long ago, Russia used a submarine to place a metal version of their national flag on the bottom of the Arctic Ocean at the North Pole, effectively trying to claim the whole region, for those who want actual examples.

    The “beauty” of this plan is that the group with the planetoids does not even have to do anything nefarious with them – they just have to show they can manipulate their celestial possessions to let others know that if they wanted to, a giant rock or rocks from space could come raining down upon their enemies if the situation ever warranted it.

    Though I have brought this idea up before here and elsewhere and either heard crickets chirping or claims of xenophobia in response, I think that someone with some actual forward-thinking minds in the US and Chinese space programs (any maybe Russia) are focusing on manned missions to the planetoids not because they are just trying to give their astronauts somewhere to go and something to do up there, but also to secure a beach head for the day when we finally do start utilizing the Sol system. National space programs have always been about nation-building and military defense first – science is a by-product and a convenient cover.

    Space is where the real growth and power will be for human civilization, though most people and politicians remain focused on the one big blue rock, even though it is a hard gravity well to escape from and the territory and resources are becoming scarcer with each passing day. Those who get the literal Big Picture will go for the space rocks first, not Mars. They will have easy access to literal tons of resources for building things like starships and a powerful weapon, just in case.